Motor vehicle systems of this nature are composed of mechanical components and control devices that specify input variables, or manipulated variables, for the system and act as a functional unit. The system responds to these input variables with output variables that are usually determined using sensors. Characteristically, however, usually only a few output values are provided, for reasons of cost. Using the measured values provided by the sensors and the computing capacity provided in the control devices, these sensors and the control devices enable the system to be monitored during operation of the vehicle. As a result, a malfunction that could endanger the safety of the operation of the motor vehicle, e.g., in the case of a fault in an anti-lock braking system, or a malfunction that results in impermissible environmental impacts, such as a fault in a fuel injection system, is recognized, and a suitable notification is provided to the driver. If the type of fault can also be determined, it is possible, using an emergency program, to maintain limited operability, for example with reduced engine output or a deactivated anti-lock braking system.
In terms of monitoring motor vehicle systems of this nature, measured signals or output variables may be checked for adherence to signal limiting values and to subject them to a plausibility check based on manipulated variable signals or input variables. If a limiting value is exceeded, an alarm message is triggered, and a standalone diagnostic device or a diagnostic device that is integrated in the control device can detect faults and distinguish between types of fault, if applicable. If more than two signals are to be evaluated jointly, model-based methods in which the model reproduces the input-output behavior of the system offer advantages. A check is run to determine whether output signals measured at the motor vehicle system match the values to be expected based on specified input signals according to the model, or if they exceed or fall below stated limiting values.
When, in addition to a model of the expected correct behavior of the motor vehicle system, models of the behavior to be expected under certain fault conditions are also provided, certain faults can be detected and localized.
The disadvantage of the foregoing approaches is that the computing complexity required to process the given quantitative signals may be considerable. Accomplishing this in real time is therefore a very complex procedure.
A method for onboard diagnosis is referred to in the publication entitled “Fault detection of a diesel injection system by qualitative modelling,” D. Foerstner, J.-Lunze, 3rd IFAC Workshop Advances in Automotive Control, pp. 273-279, Karlsruhe, 2001, this method combining a model-based diagnostic method with a qualitative modeling strategy.
Quantitative input and output signals of a dynamic system are converted to qualitative values. For this purpose, a series of threshold values up to a maximum value for the quantitative signal is specified. Each of the intervals that result is assigned to a qualitative value. If a qualitative value changes, and, therefore, the quantitative value on which it is based exceeds one of the threshold values, an event is triggered. The events are used to form an event sequence. These event sequences are compared with a complete model of event sequences. A complete model includes all possible event sequences. The state of the dynamic system can be evaluated by referring to the model.
The disadvantage of the diagnostic method made referred to in the publication is that many models of possible fault responses are incomplete and therefore cannot be used. High-frequency signal components and noise in the vicinity of threshold values can cause events to occur in rapid succession. Unnecessary computing capacity is therefore utilized without the possibility of obtaining any information as a result.